Halve, peel and dice your onion. Wash or peel and finely dice your carrot. Wash and dice your celery.
Set a large pot over high heat. Add a gloss of oil. Stir in your veg. Season well. Lower heat. Cook a few mins, till just tender.
Peel and finely chop your garlic. Cut a few sliced from your chilli (more or less, to taste) and finely chop. Add to the veg.
Pour in the balsamic vinegar. Let it cook into the veg a bit. Drain your lentils. Add them. Fill the lentil tin with water. Add it. Bring to the boil.
Crush a few of the lentils up with the back of a spoon to help thicken the soup. Lower heat. Simmer for 10 mins.
Grate the zest of your lemon into a bowl. Squeeze in 3 tbsp juice. Add 3 tbsp olive oil and a pinch of salt. Whisk till creamy. Set aside.
Taste the soup. Season as needed. Pick through the watercress, discarding (or nibbling/juicing) any tough stalks. Swirl the remaining watercress leaves through the soup right before serving. Finish with a drizzle of the lemon dressing. Garnish with slices from your remaining chilli, if you like.